Conservative rebels have warned Boris Johnson against using the overseas aid budget to pay for the transfer of up to 100m surplus vaccines to low-income countries, as they prepared for a Commons showdown over cuts to foreign aid. Rebels have threatened to humiliate the government on the eve of the British chairing of the G7 summit, which begins on Friday, by passing a technical amendment requiring the government to restore the 0.7% aid spending target next year. In an added front in their battle, they also said the aid budget should not be stretched further by being used to fund Covid vaccine doses bought commercially for UK citizens, which could now be sent to poorer countries under a G7 push. On Sunday evening dozens of rebels held firm in their demand for the government to give a clear pledge in the Commons on Monday with a timetable to restore aid spending to 0.7% of UK gross national income, in line with a Tory manifesto commitment. Andrew Mitchell, the former international development secretary who is leading the rebellion over aid being cut to 0.5% of GDP, said he was increasingly confident he had the numbers to defeat the government. “The government has three options,” he said. “They can pull the bill and that would be regarded by many as disrespectful to parliament. They can wait to see if the Speaker decides to call the amendment, and if he does then make a statement that it will accept our amendment, or lastly they can fight it out with us and if so it is a case of ‘fix bayonets’.” Mitchell writes in the Guardian that the “eyes of the world” are on the UK, and said Johnson could go into the G7 as first among equals if he restored the aid spending pledge. Johnson at the weekend urged G7 leaders to set a highly ambitious target of vaccinating the whole world by the end of 2022, including donating surplus vaccines, but the UK is one of the few G7 countries so far not to make a specific pledge to share its surplus jabs with the world’s poor. The prime minister is expected to make an announcement later this week but ministers have refused to say whether the cost of donating surplus vaccines will be funded by eating into the existing aid budget or would be classified as additional aid. The latter option could increase the overall size of the aid budget and perhaps buy off some of the potential rebels. The UK has not yet set out the price at which it will offer its vaccines to the global distribution body Covax: cost price or the market rate. Liz Sugg, the former international development minister who in November quit the government over aid cuts, welcomed Johnson’s call to vaccinate the world but said the government needed to provide “concrete numbers and dates”, adding: “The costs must be on top of the 0.5% budget so there are no further aid cuts.” Joe Biden, the US president, last week vowed to share 80m vaccines by the end of June, and UK sources said the government could match that or pledge 100m doses. In February Johnson said the majority of UK surplus vaccines would go to Covax but has said nothing more specific since. The UK pledge has partly been held up by debates over the number of surplus vaccines, the proportion that should be filtered through Covax and whether the funding for the transfer of the surplus could come from the existing aid budget or be seen as additional aid. The UK has ordered more than 500m vaccines from seven different companies, and so far 76% of the adult population have had a first dose and 51.6% have had a second. This leaves the UK with a vast stockpile of surplus jabs even if it starts vaccinating children. Some of these doses were bought at cost but others were bought at as much as £25 a shot. The World Health Organization a fortnight ago said due to the loss of expected vaccines, mainly from India, the Covax scheme urgently needed surplus doses from wealthy countries starting in June. Just over 2% of Africans have been vaccinated and African nations say the scheme has been paralysed by rich countries ordering so many vaccines. The body that classifies aid, the Organisation for Economic Co-operation and Development, is expected to say next month that the donation of surplus vaccines to low-income countries can be classified as overseas aid, but still at issue are the rates at which rich countries charge Covax for its surplus vaccines or whether they are truly donated free. As many as 40 Tory MPs from all wings of the party have been preparing to vote for an amendment on Monday that would require ministers to restore the manifesto pledge to spend 0.7% of GDP on aid from next year. Ministers cut the target to 0.5%, arguably breaching the law by not formally reporting the cut to parliament. Ministers have said the 0.7% target will be restored when economic circumstances allow, but will not set out a timetable or clarify the criteria, the key aim of the rebels. The health secretary, Matt Hancock, speaking on Sky, defended the cut. “We face a once in 300-year economic interruption, the cost of that to the exchequer has been hundreds of billions of pounds. So it is … absolutely reasonable to take the decision that the government has.”